Leak Stereo 20 stereo power amplifier
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Vintage test report


Taken from Hi-Fi News, September 1959

I AM often asked which amplifiers do I find the most interesting, those made by old-established firms or those made by newcomers to Hi-Fi? This is a difficult question to answer. In general I suppose I would find the unknown quantity of greater interest. A much easier question to answer would be, which amplifiers do I find most pleasant to use? Certainly Leak would be somewhere near the top of the list. Leak amplifiers are always a joy to handle, not only because of the high standard of performance but because of their superb workmanship and attention to detail. Here is an example: The "Stereo 20" is not only fitted with 2 extra mains sockets for gramophone motor etc., but the plugs and 2 mains leads are supplied as well. A small point perhaps, but it does simplify installation--especially for the novice. The Leak "Point one" stereo preamplifier was released late last year and is basically the twin-channel version of the popular "Point One" control unit. Although it is primarily intended for use with the "Stereo 20” or "Stereo 50", it can be used with any two amplifiers such as the TL/12, TL/15, TL/50 plus etc. The "Stereo 20" which was actually supplied with the preamplifier is essentially two 10-watt amplifiers similar to the TL/12 plus mounted on one fairly large chassis with the power supply. Grain oriented steel is used for mains and output transformers.

Circuit Details
Each channel of the preamplifier comprises 2 EF86 valves, equalising being effected by the first stage which is followed by a modified Baxandall tone control stage using the second EF86. This is followed by the volume and balance controls plus the switched condenser, which forms the rumble filter. The original Baxandall circuit gives no amplification but here the input sensitivity requirements have made it necessary to change the circuit so as to provide a worthwhile gain. Pre-set controls are fitted to all inputs except microphone and are ganged. The balance control operates on both channels simultaneously, increasing the gain of one channel whilst decreasing the gain of the other.

Each channel of the power amplifier comprises a triode stage, which is coupled to a cathode-coupled phase splitter, and an Ultra-linear output stage using 2 EL84s. The only difference from the standard Leak amplifiers is the use of a triode input stage instead of a pentode. RC smoothing is used and the common rectifier is a GZ34.

Frequency response
This was found to be within 1dB from 30 to 30,000 c/s and hum and noise was approximately -80dB relative to 15 watts. The feedback loop of 28dB could be increased to 36dB before instability became evident, thus indicating a stability factor of 8dB.

One recording characteristic is provided as shown in the diagram and CCIR 7½ ips compensation is given when playing direct from tape heads. NARTB and other characteristics can be obtained by adjusting the tone controls. The Tone Controls gave a lift of 17dB and cut of I5dB at 40 c/s with a lift of 12dB and cut of 16dB at 9 kc/c. No low-pass filter either fixed or variable is provided. The rumble filter, which is switched, attenuated from about 100 c/s with a 3dB point at 70 c/s and gives a cut-off of 13.5dB at 20 c/s. Crosstalk between channels varied from -30dB at 10 Kc/s to -54dB at 1,000 c/s. Matching was in general within 1dB for all settings of the controls. The balance control is ganged and either channel can be faded to zero without affecting the other.

The sensitivity was ample for the Decca stereo pick-up to be used successfully and good results were also obtained with the Tannoy "Vari-twin." Ample power was available for two Electrostatic speakers in a fairly large room. For those who really need even greater output there is the "Stereo 50" which is rated at some 25 watts per channel and is directly interchangeable with the more modest "20". (Relatively modest that is!) I must confess that I missed the filter and slope facilities, which are such a feature of the Varislope III reviewed in February. Perhaps a stereo version is contemplated—I am sure many people will find the extra cost worthwhile. After all, distortion is probably a bigger problem with stereo programme sources than with monophonic . . . The styling and presentation of the control unit is similar to the Varislope—a transparent Diakon plastic panel finished in maroon, cream and gold with elegant knobs to match. The wiring and general engineering is, as usual, impeccable. Summing up then, this stereo combination is excellent value for money and can be confidently recommended.